Prostheses for heart assistance or replacement are being used more and more to back up or to replace a failing human heart. For assistance purposes, a prosthesis is frequently used to back up half of the heart, the left half or the right half, whereas for replacement purposes, a prosthesis is used that is constituted by two artificial half-hearts, Such a prosthesis always includes at least one pump, an electrical or pneumatic pump drive system, and means for controlling the energy delivered to the drive system to obtain appropriate operation of the prosthesis.
The type of pump used may be a positive displacement pump such as the rotary piston pumps known as the Wankel engine pump or the Cora pump as described in patent FR 2 389 382, membrane pumps, piston pumps, or centrifugal pumps.
The words "pump" and "prosthesis" are used interchangeably below in this document to designate the mechanical device that sucks in and expells the blood of the patient.
In general, such pumps provide a periodic or pulsed flow.
Unless specified otherwise, the term "period" is used below in this document to designate the time interval between the blood flow conveyed by the pump passing on two successive occasions through the same value and in the same direction.
Generally this variation period in the blood flow conveyed by the pump is equal to or is a fraction of the period of the rotary motion of a rotor for rotary pumps or of the translation motion of a piston for piston pumps or membrane pumps.
Pumps are often connected in parallel with a heart or a half-heart that continues to function. Connection to the implantation sites is performed by means of cannulas.
The inlet or admission of the pump is thus connected to the pulmonary or systemic venous return, sometimes via the auricle of the patient's heart, sometimes via the ventricle of the patient's heart.
The flow rate of a prosthesis cannot exceed the flow rate of blood available in the venous return, otherwise there would be a collapse of the blood vessels or a deformation of the valve-forming heart wall or vascular wall, which phenomena must be avoided absolutely.
Devices are known that use pressure sensors for monitoring proper filling of the implantation site from which the prosthesis takes blood by comparing the measured pressure with a reference pressure.
Patent FR 2 470 593 describes a regulator device for a heart prosthesis that includes a plurality of blood pressure servo-controls performed by comparison with a reference pressure.
Those devices measure a mean pressure and are sensitive to variations in physical parameters that may give rise to variations in the measured magnitude without being indicative of a prosthesis misfunction. The parameters that influence these measurements are, in particular, surrounding or atmospheric pressure, and accelerations due to shocks, transport, motion, . . . .
The pressures in the various heart cavities are relative: the reference pressure is the intrathoracic pressure which is itself influenced by the physical environment.
In addition, the amplitude of the pressure variations to be monitored is small compared with the measured magnitude and requires frequent and expensive calibration.
For example, the amplitude of auricular pressure pulses of the right heart lies in the range 0 to about 4 millibars, and for the left heart in the range 0 to about 10 millibars.
It is also known that the heart rate of a patient varies over time, in particular during the day as a function of the patient's metabolism.
Consequently, the drive speed of a heart prosthesis must be capable of adapting to slow changes in heart rate corresponding to said variations in the patient's metabolism.
Until now, these variations have not been monitored by the regulation apparatus, and the drive speed of the prosthesis has been adapted manually and empirically by modifying the reference drive speed.
The problem to be solved is to provide reliable regulation methods and apparatuses for adapting the flow rate conveyed by heart prostheses that provide a periodic flow to the rate at which blood fills the implantation site, such methods and apparatuses being very insensitive to the effects of environmental physical parameters such as atmospheric pressure, or acceleration, and not requiring frequent calibration.
The problem to be solved is also to provide a regulation method and apparatus for a heart prosthesis that is capable of adapting the drive speed of the prosthesis to variations in the metabolism of the patient.